1. Field of the Invention
The present invention relates to an apparatus for magnifying an image. More particularly, the present invention relates to a behind-the-lens, wafer thin reading telescope for use with spectacles or eyeglasses, the telescope having a relatively long optical path through the optical system while at the same time having a relatively short dimension from front to back. This wafer thin reading telescope aids partially sighted patients or users especially in the reading of print.
2. Description of the Prior Art
A large of number of people have impaired vision that cannot be corrected with spectacles. As reported in Vision Research--A National Plan 1983-1987: The 1983 Report of the National Advisory Eve Council, "[a]n estimated ten million Americans have irreversibly impaired vision. About 1.5 million Americans are unable to read ordinary newsprint, even with the best possible optical correction, and five hundred thousand people are classified as legally blind." These legally blind people have a visual acuity of about 20/200 or worse. The visual acuity of the other low vision people is not correctable to better than 20/50 with single lens systems such as conventional eyeglasses or contact lenses, and correction to a range of 20/100 is more typical. This range of correction leaves the person with great difficulty in reading signs, distinguishing the shapes of objects and reading print.
These partially sighted people, and especially the elderly among them, want to be able to read. In a recent survey of low vision patients, two-thirds of them stated that reading was the number one task that they wanted to achieve.
The most common optical aids to help in reading are spectacle magnifiers, stand and hand magnifiers, and reading telescopes. The large disadvantage with spectacle magnifiers is the close working distance. A plus 12 diopter microscopic glass spectacle magnifier magnifies 3 times, but has only a three inch working distance. This distance is much too close for reading comfort for many patients. Stand and hand magnifiers give a larger working distance but a smaller field of view and do not allow the user's hands to be free.
The other common optical aid is the reading telescope. Conventional reading telescopes have the disadvantage of a smaller field of view than spectacle magnifiers and very poor cosmetic appearance. However, they do offer a more normal working distance such as 12 inches. Reading telescopes small enough to be attached to the lens of a pair of eyeglasses have been known in the art for many years. Most such devices suffer the disadvantage that they are secured to the eyeglasses lens by extending through a hole drilled through the lens and extend forward from the eyeglasses lens for a distance of up to four inches. Most of these devices are heavy and are immediately noticed by others so as to be cosmetically undesirable. Currently available bioptic telescopes are either Galilean or Keplerian telescopes that are generally cylindrical in form and have both objective and eyepiece lenses oriented along the axis of a cylinder. These telescopes range in size from about 1 to about 4 inches in length and have magnifying powers ranging from about 2.times. to about 8.times..
Many types of bioptic telescopes are known in the art and are prescribed on an individual basis for a patient depending on the patient's specific needs. Bioptic telescope systems are prescribed for patients desiring increased visual acuity for reading or close work. Monocular telescope systems can be prescribed for patients having reduced vision in one eye only or for a patient having vision in only one eye.
Typically, these bioptic telescopes are mounted to eyeglasses by boring a hole of appropriate size through the carrier lens, either immediately above or below the normal line of sight, and securing the telescope in the hole. Both head and eye movements are required to use a bioptic telescope conventionally mounted in an eyeglasses lens. For example, if a telescope is mounted above the line of sight, the patient first rotates the head downward to direct the telescope at the object the patient wishes to see, and then the patient rotates the eyes upward by a corresponding amount to bring the visual axis of the eye into coincidence with the optical axis of the telescope. Many more reading telescopes could be prescribed if patients did not reject them because of their appearance, weight, cost and inconvenience.
Other types of telescope systems have been devised with the purpose of reducing their weight and unwieldy size and to reduce the forward protrusion in front of the carrier lens. Examples of these types of systems include the Galilean BITA lens manufactured by the Edwards Optical Corporation of Virginia Beach, Va. This cylindrical telescope system is mounted through and behind the eyeglasses lens directly in front of the eye of the wearer. Although this system provides desired magnification, because it is miniaturized and because it is Galilean, the lens of the system must be mounted very close to the patient's eye in order to provide a good field of view. This creates the potential for eye injury. In addition, when the patient blinks, eyelashes invariably brush over the lenses.
Another telescopic lens system available in the prior art is a "bioptic periscope" invented by the inventor of the present invention and the subject of U.S. Pat. No. 4,795,235 issued Jan. 3, 1989 (the '235 patent). The invention in the '235 patent involves the use of a roof prism which enables the bioptic apparatus to be mounted entirely behind an eyeglasses lens being laterally displaced from a patient's straight ahead gaze. The apparatus is flush mounted within a hole formed in the carrier lens such that there is no forward protrusion from the front surface of the carrier lens. The apparatus is placed at an approximately 36-degree viewing angle relative to the straight ahead gaze. To utilize the bioptic periscope, the patient is required to rotate the eyes temporally in order to bring the pupil into axial alignment with the eyepiece lens of the bioptic periscope.
By having the bioptic periscope displaced from the normal straight ahead gaze, the blind area or "blind spot" created in the peripheral field surrounding prior art bioptic apparatus is eliminated or reduced. In addition, because the apparatus is laterally displaced at a point off the visual axis, the danger present by having the eye of the patient in close proximity to the apparatus is greatly reduced or eliminated. The bioptic periscope of the '235 patent is a Keplerian telescope. In other words, the apparatus utilizes plus lenses as the objective lens and the eyepiece lens. By having two plus lenses, a greater degree of magnification can be achieved.
In the invention of the '235 patent, light reflected from an image passes through an objective lens and through an apex face of the roof prism. The light traverses the prism and is internally reflected from another apex face of the roof prism. The light traverses the prism a second time and is reflected from the roof of the prism. The roof reflects the light back to the first apex face where the light is reflected a third time causing the light to traverse the roof prism a fourth time. The light is then transmitted through the roof prism face and through the eyepiece lens where it is received within the patient's pupil. Wedge prisms can be mounted within the apparatus between the apex faces of the roof prism and the eyepiece lens to provide additional bending of light to bring it more precisely into coincidence with the visual axis of the patient's eye. In this apparatus, the light traverses the roof prism four times. These traverses increase the focal length of the bioptic periscope sufficiently to allow magnification of an image in a range from about 3.times. to about 6.times. depending on the powers of the objective and eyepiece lenses used.
The focal length of the bioptic periscope of the '235 patent is in the range of about 1 foot to infinity. This vision aid can be used for reading, but only for short periods of time as the eye would get tired using the 36 degree eye rotation for prolonged reading. This vision aid is only used monocularly. Though this vision aid is compact, it is a Keplerian system and requires a roof prism which increases its size. Therefore, this vision aid is entirely unsuitable for normal, prolonged reading conditions.